At recent, a ternary thin film such as CuInSe2 (hereinafter, referred to as ‘CIS’) or CuIn1-xGaxSe2 (hereinafter, referred to as ‘CIGS’) as one of compound semiconductor has been actively researched. Unlike silicon solar cells in the related art, the CIS-based thin film solar cells can be manufactured with a thickness of 10 microns or less and have stable characteristics in long-term use. In addition, according to experiments, maximum conversion efficiency thereof is 19% which is far superior to other solar cells, so that the CIS-based thin film solar cells can be highly expected to be commercialized as low-cost, high-efficiency solar cells as a substitute for silicon solar cells.
Accordingly, as a representative method of manufacturing a CIGS thin-film, there are a co-evaporation method, a selenization method of injecting copper, indium, and gallium on a specimen and performing annealing using H2Se or element selenium.
As disclosed in U.S. Pat. No. 4,523,051, in a co-evaporation method, metal elements are simultaneously evaporated and deposited under a vacuum ambience. Although high efficiency of 20% is obtained in the laboratory, the aforementioned co-evaporation method has problems of low material utilization and high cost apparatuses, difficulties in large-area deposition, relatively low throughput, and the like, which are to overcome for commercialization thereof.
As disclosed in U.S. Pat. No. 4,798,660 or 5,141,564, in a selenization method using annealing, the CIGS thin-film is formed by performing thermal treatment on a substrate on which CIG, ICG, and CGI thin films are formed under an H2Se or element selenium vapor ambience. Among these type methods, a method using annealing under an ambience of selenium containing gas such as H2Se, a method of depositing selenium on a specimen where copper, indium, gallium, and the like is coated and heating to allow selenium to react in a liquid phase, and the like are expected to be commercialized in the near future. In the method of depositing selenium on a specimen where copper, indium, gallium, and the like is coated and heating, during a thermal treatment process, the selenium coated or deposited on the uppermost layer is liquefied, and the liquefied selenium reacts with copper, indium, gallium to generate the CIGS thin film. Due to much higher density of element selenium than that of a gas, the reaction time can be shortened and high-quality CIGS can be obtained. Therefore, several companies have adopted the aforementioned method in mass production. The core of this process is to minimize evaporation of liquefied selenium before the reaction of liquefied selenium, to maintain selenium vapor pressure during the reaction, and to maintain uniform reaction to prevent bowling dewetting, or the like. However, in order to solve problems of surface tension due to viscosity of liquefied selenium, non-uniform evaporation due to inevitable non-uniformity of temperature, damage to molybdenum electrode due to diffusion of liquefied selenium into a molybdenum electrode, and the like, a temperature of a thermal treatment process, evaporation flux, and other complex factors needs to be very accurately controlled. Thus, the above-mentioned methods have technical difficulty in commercialization. If a method which does not depend on the reaction of liquefied selenium is contrived, the aforementioned problems may be solved more easily and economically.
On the other hand, as a method capable of avoiding the reaction of liquefied selenium and increasing the selenium vapor pressure, in a method in the related art illustrated in FIG. 1 where RTP is performed under a H2Se or dimethyl selenium vapor ambience, an ambience of a relatively high pressure of 1/50 atm or more can be easily maintained, so that reactivity can be improved. Therefore, the method has been adopted in mass production. However, the method has problems of high cost and toxicity of reaction gases to be solved.
As one of selenization methods using annealing, a method in the related art illustrated in FIG. 2, where element selenium and a specimen where copper, indium, and gallium thin films are deposited on an electrode are simultaneously heated and, after that, selenium vapor is supplied to generate a light absorbing layer, is very economical, but the method is not suitable for obtaining high efficiency.
The aforementioned methods have been tried so far due to many advantages of low cost manufacturing, convenience of processes, and non-toxicity which are useful for commercialization and for the purpose of researches preceding researches for other methods or researches for CIGS reaction mechanism. However, there is no report of achievement of manufacturing of solar cells having high efficiency of about 10% through the aforementioned methods. If a method capable of obtaining high efficiency is contrived, the method will be a new method suitable for mass production of CIGS solar cells.
On the other hand, in the method illustrated in FIG. 2, where element selenium and a specimen are deposited on a CIG/CI thin film electrode are simultaneously heated and, after that, selenium vapor is supplied to generate a light absorbing layer, the selenium vapor pressure in the chamber cannot be controlled to be sufficiently high, so that high efficiency cannot be obtained. Thus, if a sufficient density of selenium in the chamber can be maintained, the selenization method using an element selenium vapor may be adapted to easily obtain large area, high efficiency, and high productivity.